Acoustic borehole logging instruments are used to measure velocities of earth formations in one or more modes of acoustic energy propagation. Acoustic borehole logging instruments are typically used in liquid-filled boreholes drilled through the earth formations. Velocity is generally determined using these instruments by measuring the time taken by an acoustic energy pulse to traverse a particular distance along the wall of the wellbore. The wall forms the interface between the liquid in the wellbore and the earth formations.
One form of acoustic energy pulses of particular interest to the disclosure is referred to as “Stoneley” waves. Stoneley waves are essentially guided interface waves traveling in the borehole at the fluid-formation interface. It had been determined in earlier research that a relationship is likely to exist between the transmission properties of Stoneley waves as they propagate along the borehole, and the hydraulic characteristics of the formations along the wellbore wall. U.S. Pat. No. 5,784,333 to Tang et al., having the same assignee as the present disclosure teaches a method for determining the permeability of earth formations penetrated by a borehole from acoustic signals measured by an acoustic borehole logging instrument. The method includes separating components from the measured acoustic signals which represent Stoneley waves propagating through the earth formations. Signals representing Stoneley waves propagating through the same earth formations are synthesized. The separated acoustic signal components and the synthesized Stoneley wave signals are compared. The permeability is determined from differences between the synthesized Stoneley wave signals and the separated acoustic signal components. Hornby et al. (1989) discloses a method of evaluating fracture arrivals in an earth formation by analyzing the Stoneley wave signals.
Other methods of estimating formation permeability have made use of the electroseismic effect wherein an electric field is produced in an earth formation by generating a seismic wave in a borehole, and the resulting electric field is measured. Due to the fact that the seismic wave is omnidirectional, the resolution of the method is poor. The present disclosure addresses this drawback.